Method for the thermal stabilisation of fibres and said type of stabilised fibres

ABSTRACT

The invention relates to a method for the production of thermally stabilised melt spun fibres, in which polyacrylonitrile (PAN) fibres or PAN fibre precursors produced by melt spinning are treated in an aqueous alkaline solution which comprises in addition a solvent for PAN. Likewise, the invention relates to fibres which are producible according to this method.

The invention relates to a method for the production of thermally stabilised melt spun fibres, in which polyacrylonitrile (PAN) fibres or PAN fibre precursors produced by melt spinning are treated in an aqueous alkaline solution which comprises in addition a solvent for PAN. Likewise, the invention relates to fibres which are producible according to this method.

Carbon fibres which are gaining ever greater importance in the field of technical fibres are produced, according to the state of the art, by thermal conversion of separately produced precursor fibres. Materials for the precursor fibres are above all PAN (co)polymers (acrylic precursors) and also pitch. Acrylic precursor fibres have been produced commercially to date exclusively via wet- or dry-spinning methods. For this purpose, a solution of polymers with concentrations ≦20% are spun either in a coagulation bath or a hot steam atmosphere, the solvent diffusing out of the fibre. In this way, qualitatively high-value precursors are produced, however, the costs of the methods are comparatively high. This results, on the one hand, from the required solvents and handling thereof, on the other hand, from the relatively low throughput in solution spinning methods.

Because of the strong inter- and intramolecular interactions of the nitrile groups, the melting point of PAN at 320° C. is above the decomposition temperature of the polymer. This means that melt spinning of pure PAN is not possible, the polymer does not behave as a thermoplast but rather as a duroplast. At the same time, the possibility for the production of precursor fibres by means of melt spinning would however imply a significant cost saving in the precursor production since the throughput during melt spinning is substantially higher and in addition no solvents incur which cause costs for purchasing and recycling/disposal.

Efforts have been made for several decades to make PAN amenable to processing by means of melt spinning. In principle, approaches by way of an external softening (mixing of the polymer with additives) and internal softening (copolymerisation) must thereby be differentiated. In both cases, the interaction of the nitrile groups is thereby disturbed so that the melting is effected below the decomposition temperature of the polymer.

An essential prerequisite for the further processing to form carbon fibres is the possibility of stabilising the fibres subsequently oxidatively. This process is implemented at temperatures above 200° C. and results in the formation of cyclic structures which firstly enable the subsequent carbonisation. Of course, this can only succeed when the fibres do not melt at the stabilisation temperatures—which represents an additional problem to be solved since the stabilisation temperatures are in general higher than the processing temperatures during melt spinning.

As already mentioned, the pre-stabilisation of melt spun PAN fibres against the actual oxidative stabilisation/carbonisation is one of the crucial problems which needs to be solved in the context of melt spinnable PAN precursors. The stabilisation is normally implemented in the temperature range between 200° C. and 300° C. under air atmosphere. It is obvious that the above-described melt spun PAN precursor fibres soften/melt under these conditions and hence would make a stabilisation with simultaneous maintenance of the fibre properties impossible. This means that the fibre has to be put firstly into an unmeltable state after spinning.

The only path shown to be as practicable to date for this purpose in the literature is the crosslinking of the finished fibre by UV- or electron radiation (Mukundan et al.; Polymer 2006 47: 4163-4171), the former demanding the incorporation of an additional photosensitive comonomer, the second sensitisation of the polymer to the electron beams for example by an introduced crosslinker (triallylisocyanurate or comparable). The disadvantages of the methods are obvious; apart from the commercial availability of UV-active monomers which in practice does not exist, further imperfections are introduced into the fibre which have a negative effect on the properties of the resulting carbon fibre.

Starting herefrom, it was the object of the present invention to optimise the thermal stability of melt spun acrylic fibres such that the fibres, after spinning, are present in an unmeltable state for further processing.

This object is achieved by the method having the features of claim 1 and also by the melt spun fibres having the features of claim 14. The further dependent claims reveal advantageous developments.

According to the invention, a method for the production of thermally stabilised, melt spun acrylic fibres is provided, in which a fibre produced by melt spinning or a fibre precursor is pre-stabilised with a mixture comprising a solvent for PAN, in particular selected from the group consisting of dimethylsulphoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium rhodanide solutions and mixtures hereof and an aqueous alkaline solution.

Surprisingly, it was established that a fibre produced by melt spinning can be converted into an unmeltable state by treatment with such a solution.

It is thereby preferred that the mixture comprises from 0.1 to 60% by volume of the PAN solvent and from 40 to 99.9% by volume of the aqueous alkaline solution.

The pre-stabilisation is preferably implemented in a modification bath comprising the mixture at a temperature of 20 to 80° C., preferably of 40 to 65° C., within a dwell time of 5 s to 2 min, preferably of 10 s to 60 s.

Preferably, the aqueous alkaline solution comprises from 3 to 15 mol/l of at least one alkali- or alkaline earth hydroxide, preferably an alkali hydroxide, particularly preferably potassium hydroxide or sodium hydroxide.

The ratios of PAN solvent and aqueous alkaline solution in the mixture are preferably adjusted as a function of the titre of the corresponding fibre.

Preferably an oxidative stabilisation follows the pre-stabilisation. The oxidative stabilisation is thereby implemented preferably at temperatures of 200 to 350° C. in an oxygen- or air-containing atmosphere.

According to the invention, the fibres are preferably carbon fibres, the fibre precursors consisting of a copolymer of polyacrylonitrile or essentially comprising this.

It is thereby preferred that the fibre precursor is producible by a method in which

-   -   i. a copolymerisation of 95 to 80% by mol of acrylonitrile with         at least one comonomer selected from         -   a) 5 to 20% by mol of at least one alkoxyalkylacrylate of             general formula I

-   -   -   with         -   R=C_(n)H_(2n+1) and n=1-8 and m=1-8, in particular n=1-4 and             m=1-4         -   b) 0 to 10% by mol of at least one alkylacrylate of general             formula II

-   -   -   with         -   R=C_(n)H_(2n+1) and n=1-18,         -   c) 0 to 10% by mol of at least one vinyl ester of general             formula III

-   -   -   with         -   R=C_(n)H_(2n)1 and n=1-18,         -   is implemented in the presence of at least one initiator and

    -   ii. the copolymer is spun with an extruder with at least one         nozzle, suitable for spinning, at the extruder outlet to form         mono- or multifilaments.

The copolymer preferably has a melt viscosity which remains constant or decreases with increasing temperature up to 240° C., in particular up to 260° C.

It is further preferred that 8 to 12% by mol of the comonomer in a) and/or 1 to 5% by mol of the comonomer in b) and/or 1 to 5% by mol of the comonomer in c) are present.

The copolymerisation can thereby be effected preferably by precipitation polymerisation, emulsion polymerisation and/or polymerisation in a solvent.

The solvent is preferably selected from the group consisting of dimethylsulphoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium rhodanide solution and mixtures hereof.

According to the invention, fibres which are producible according to the previously described method are likewise provided. This hereby concerns in particular carbon fibres.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject according to the invention is intended to be explained in more detail with reference to the subsequent examples and Figures, without wishing to restrict said subject to the specific embodiments shown here.

FIG. 1 shows, with reference to a schematic diagram, the temperature dependency of the storage modulus of standard non-meltable acrylic fibres and meltable acrylic precursors before the treatment according to the invention.

FIG. 2 shows, with reference to a diagram, the temperature dependency of the storage modulus of an untreated fibre

FIG. 3 shows an IR spectrum of an untreated fibre

FIG. 4 shows, with reference to a diagram, the temperature dependency of the storage modulus of a fibre treated according to the invention

FIG. 5 shows an IR spectrum of a fibre treated according to the invention

FIG. 6 shows a photo of an untreated (at the top) and a multifilament (at the bottom), treated according to the invention, made of 42 fibres which were both subjected to an oxidative stabilisation at 230° C. Melting of the untreated filament can be clearly recognised, the treated filament melts and does not bond.

The effect of the treatment is described with the following tests:

1. Bonding of the Fibres:

Approx. 500 mg fibre short section is placed between aluminium foil and incubated in the drying cabinet with loading by a weight of approx. 100 g at various temperatures respectively for 10 min. It is recorded from what temperature the result is bonding of the aluminium foil by the fibres. This temperature corresponds to a softening- or melting temperature.

2. Melting Table Microscopy:

The thermal behaviour of the fibres is observed under the melting table microscope. It is recorded whether melting of the fibres can be detected.

3. Solubility of the Fibres:

Approx. 500 mg fibre short section is stored in 10 ml MDSO and it is noted after what time the fibres dissolve. Crosslinkings are detected with this test method.

4. Dynamic-Mechanical Analysis (DMA):

The storage modulus of a fibre of 10 cm length is determined as a function of the temperature. FIGS. 2 and 4 describe the basic course of the curves. Non-modified PAN (not meltable) shows a loss of the storage modulus with the temperature up to approx. 140° C., thereafter a constant residual storage modulus is maintained which ensures the mechanical stability of the fibre. Meltable PAN, which was not treated, shows a loss of the storage modulus up to approx. 80° C., at this temperature the modulus has decreased to 0. Above 80° C., the fibre is so soft that it is no longer mechanically loadable and tears (see FIG. 1).

EXAMPLES Comparative Example Untreated Melt Spun Fibre

An untreated melt spun fibre made of a PAN copolymer with 10% methoxyethylacrylate and a molar mass of 15,000 g/mol, individual fibre titre 0.82 tex, was examined.

It was thereby shown during the bonding test that the fibres bond from 80° C. On the melting table microscope, complete melting at 185° C. could be observed. During the solubility test, the fibres were completely dissolved within 2 min. During the DMA, the modulus decreases, up to 100° C., to 0 (see FIG. 2).

From the IR spectrum (FIG. 3), an ester grouping at 1,730 cm⁻¹ can be clearly detected, just as a nitrile group at 2,240 cm⁻¹. Further characteristic bands are not present.

Example Melt Spun Fibre Treated According to the Invention

A melt spun fibre treated according to the invention, made of a copolymer with 10% methoxyethylacrylate and a molar mass of 15,000 g/mol, individual fibre titre 0.82 tex, was examined. The fibre described in the comparative example was, for this purpose, placed in a modification bath of the composition 50% DMSO and 50% 4.5 M aqueous KOH for 60 s at 70° C. The fibre discolours to reddish brown. Subsequently, it is washed neutrally and dried in a vacuum at 50° C.

During the bonding test, it was shown that the fibres do not bond up to a temperature of 230° C. On the melting table microscope, no melting of the fibres could be observed up to 250° C., merely the colour of the fibres changed from reddish via reddish brown to black. The result of the solubility test is that the fibres are maintained entirely after 24 h. During the DMA, the modulus decreases to 250° C., but still has a value greater than 0 even at this temperature (see FIG. 4).

From the IR spectrum (FIG. 5), it results, in comparison to the IR spectrum in FIG. 3, that the intensity of the band of the ester grouping at 1,730 cm⁻¹ has decreased significantly, whilst the intensity of the band of the nitrile group is constant at 2,240 cm⁻¹. Furthermore, a band which can be ascribed to the formation of the PAN conductor structure occurs at 1,570 cm⁻¹. 

1-15. (canceled)
 16. A method of thermally stabilising melt spun acrylic fibres in which an acrylic fibre produced by melt spinning or a fibre precursor, which involves pre-stabilising the acrylic fibre produced by melt spinning or the fibre precursor with a mixture comprising a solvent for polyacrylonitrile and an aqueous alkaline solution.
 17. The method according to claim 16, wherein the mixture comprises from 0.1 to 60% by volume of the solvent and from 40 to 99.9% by volume of the aqueous alkaline solution.
 18. The method according to claim 16, wherein the solvent is selected from the group consisting of dimethylsulphoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium rhodanide solutions, and mixtures thereof.
 19. The method according to claim 16, wherein the pre-stabilisation is effected in a modification bath comprising the mixture at a temperature of 20 to 80° C., within a dwell time of 5 s to 2 min.
 20. The method according to claim 16, wherein the aqueous alkaline solution comprises from 3 to 15 mol/l of at least one alkaline earth- or alkali salt.
 21. The method according to claim 16, wherein the proportions of solvent and aqueous alkaline solution in the mixture are adjusted as a function of the titre.
 22. The method according to claim 16, wherein an oxidative stabilisation and carbonisation under inert gas, at temperatures of 800 to 1,700° C. follows the pre-stabilisation.
 23. The method according to claim 22, wherein the oxidative stabilisation is effected at a temperature of 200 to 350° C. in an oxygen- or air-containing atmosphere.
 24. The method according to claim 16, wherein the fibre precursor is produced by a method in which i. a copolymerisation of 95 to 80% by mol of acrylonitrile with at least one comonomer selected from a) 5 to 20% by mol of at least one alkoxyalkyl acrylate of the general formula I

 with  R=C_(n)H_(2n+1) and n=1-8 and m=1-8, b) 0 to 10% by mol of at least one alkylacrylate of the general formula II

 with  R=C_(n)H_(2n+1) and n=1-18, c) 0 to 10% by mol of at least one vinyl ester of the general formula III

 with  R=C_(n)H_(2n+1) and n=1-18, is implemented in the presence of at least one initiator and ii. the copolymer is spun with an extruder with at least one nozzle at the extruder outlet to form mono- or multifilaments.
 25. The method according to claim 24, wherein the copolymer has a melt viscosity which is constant or decreases with increasing temperature up to 240° C.
 26. The method according to claim 24, wherein 8 to 12% by mol of the comonomer in a) and/or 1 to 5% by mol of the comonomer in b) and/or 1 to 5% by mol of the comonomer in c) are present.
 27. The method according to claim 24, wherein the copolymerisation is effected by precipitation polymerisation, emulsion polymerisation and/or polymerisation in a solvent.
 28. The method according to claim 27, wherein the solvent is selected from the group consisting of dimethylsulphoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene carbonate, propylene carbonate, aqueous sodium rhodanide solution and mixtures thereof.
 29. A melt spun fibre stabilised thermally according to the method of claim
 16. 30. The melt spun fibre according to claim 29, which is further processed to form a carbon fibre by an oxidative stabilisation and carbonisation under inert gas, at a temperature of 800 to 1,700° C. 